In general, cutting of a thin sheet of brittle material such as glass and ceramics is often accomplished by sequentially performing a scribe process and a break process. The scribe process is a process to form a very shallow scribe groove together with a short crack (vertical crack) extending vertically from a bottom of the scribe groove by moving an abacus ball-like wheel cutter and the brittle material relatively to each other while pressing the wheel cutter against the brittle material. For example, when the glass sheet is 0.7 mm thick, a bite depth of the wheel cutter is set to approximately 5 μm to form a scribe groove having a width of approximately 10 μm and a vertical crack having a depth equal to approximately 20% of the thickness of the glass sheet (approximately 130 μm). The break process is a process to divide the sheet after the scribe process by bending it along the scribe groove or pressing rubber against it to cause the vertical crack in the scribe groove to reach the backside of the sheet.
As is well known to those skilled in the art, a liquid crystal display plate is configured by bonding together two glass sheets through seal material, and one of the glass sheets has a circuit element pattern formed thereon and the other has a color filter pattern formed thereon. In manufacturing of the liquid crystal display plates, a many-at-once production approach is generally adopted, that is, circuit patterns for a plurality of crystal liquid display plates are formed on a mother glass sheet, color filter patterns for the plurality of crystal liquid display plates are formed on another mother glass sheet, these two mother glass sheets are bonded together through seal materials which are respectively formed for crystal liquid display plates, and finally each crystal liquid display plate is cut off from the bonded mother glass sheets. When cutting off from each crystal liquid display plate, the glass sheet with the circuit pattern may be sometimes cut at a location slightly different from that on the other glass sheet with the color filter pattern because of taking out of wiring. Therefore, it is not easy to accomplish the break process by bending or folding the glass sheets to cut off each liquid display plate because the two glass sheets are bonded together and some of their cutting locations are slightly different from each other.
As described in JP-A-6-48755 specification, a method for dividing two bonded glass sheets has been proposed, wherein a rubber plate and the like is pressed against the bonded glass sheets formed by sticking two glass sheets together from the backside of a scribe groove to extend a vertical crack in the scribe groove. This method is applicable to such two bonded glass sheets as well as to those having slightly different cutting locations because it does not cause remarkable bending deformation in the glass sheets. However, this method may have an impact on the glass sheets when the rubber plate is pressed against them from the backside of the scribe groove. Thus, such an impact may be likely to cause some chipping or crack in the glass sheets or to break the wiring, degrading the performance of the glass sheets in a final product.
As described in JP-A-9-12327 specification, another method for dividing such glass sheets is known to those skilled in the art, wherein the division is accomplished without any bending deformation in or any impact on the glass sheets by irradiating the surface of the glass sheets with laser light to heat the surface of the glass sheets locally and then quenching the glass sheets. According to this method, a heated area is quenched to cause some tensile stress in a quenched scribe groove and then a vertical crack in the scribe groove is extended to allow the glass sheets to be divided. This method can suppress some factor for performance degradation of the glass sheets in a final product because it does not cause any impact on or bending deformation in the glass sheets. In addition, this method is characterized by direct irradiation of laser light on the scribe groove for heating. When the scribe groove is mechanically formed with a wheel cutter or the like, the edge of the wheel cutter may exert pressure on the scribe groove to cause distortion around the scribe groove, because the wheel cutter is kept pressed against the glass sheets during the groove formation. When the scribe groove is formed with laser light, the heat of laser light may cause distortion around the scribe groove. Thus, if an amount of heat is applied to the scribe groove by directly irradiating, it with laser light, distortion may be increased to cause some crack in an in-plane direction in the glass sheets or cause some glass piece to be peeled off along the scribe groove and these phenomena may degrade the quality of the divided glass sheets. In particular, for an improved throughput of the break process, laser light must be moved along the scribe groove at a higher speed. However, if laser light is simply moved at a higher speed, the amount of heat to be applied to the glass sheets through laser irradiation will be decreased with no tensile stress leading to division. Therefore, the irradiation output of laser light must be increased to secure a required amount of heat at a higher moving speed but such an increased laser irradiation output will cause the distortion around the scribe groove to affect the glass sheets, resulting in some crack in the in-plane direction in the glass sheets or some glass piece to be peeled off along the scribe groove.
Also, as described in JP-A-7-328781 specification, a method to form a scribe groove by applying two lasers to a glass sheet in the vicinity of a scribe groove predetermined line is known to those skilled in the art. Similarly to the above-mentioned method wherein the glass sheet is irradiated with laser light to form the scribe groove, this method forms the scribe groove by applying some heat to the glass sheet surface around the scribe groove predetermined line to cause some tensile stress there. However, this method differs from the above-mentioned method in that the scribe groove predetermined line is not directly irradiated with laser and thus a less amount of heat is applied to the scribe groove predetermined line, so that any heat influence on workpieces can be reduced and the working speed can be increased by two to five times as compared with the above-mentioned method of forming a scribe groove through direct laser irradiation.
However, similarly to the above-mentioned laser irradiation, this method also irradiates the glass sheets locally with laser and forms the scribe groove while moving. Thus, the laser moving speed for the “scribe groove forming process” can be increased by approximately five times but these two methods of using laser may cause the following problem when they are simply applied to the “cutting process” for an improved throughput. That is, the speed of laser light which moves along the scribe groove predetermined line in the cutting process must be increased to improve the throughput but if the moving speed of laser light is simply increased, the amount of heat to be applied to the glass sheets will be decreased, resulting in no tensile stress which leads to scribe groove formation and eventually no cutting. To avoid this disadvantage, the irradiation output of laser light must be increased to secure a required amount of heat and to keep a higher moving speed but similarly to the above-mentioned method of laser irradiation, such an increased laser irradiation output will cause the distortion around the scribe groove to affect the glass sheets, resulting in some crack in the in-plane direction in the glass sheets or some glass piece to be peeled off along the scribe groove.
As described above, the prior art described in JP-A-6-48755 specification has the disadvantage that it may have an impact on the glass sheets when the rubber plate is pressed against them from the backside of the scribe groove and thus, such an impact may be likely to cause some chips or cracks in the glass sheets or to break the wiring, degrading the performance of the glass sheets in a final product. Another prior arts described in JP-A-9-12327 specification and JP-A-7-328781 specification have the disadvantage that the irradiation output of laser light is forced to be increased for an improved throughput in the “cutting process” and thus, such an increased laser irradiation output will cause the distortion around the scribe groove to affect the glass sheets, resulting in some crack in the in-plane direction in the glass sheets or some glass piece to be peeled off along the scribe groove and degrading the performance of the glass sheets in a final product.
Particularly, in manufacturing of liquid crystal display plates, a many-at-once production approach is generally adopted, that is, circuit patterns for a plurality of crystal liquid display plates are formed on a mother glass sheet, color filter patterns for the plurality of crystal liquid display plates are formed on another mother glass sheet, these two mother glass sheets are bonded together through seal materials which are respectively formed for the crystal liquid display plates, and finally each crystal liquid display plate is cut off from the bonded mother glass sheets. With a recent increase in demand for liquid crystal display plates, the number of liquid crystal display plates to be cut off at a time is increased and the image display section for each liquid crystal display plate is also increased in size, resulting in larger and larger mother glass sheets. This leads to a significantly increased cutting speed with respect to the length of cutting each liquid crystal display section in the cutting process. However, the prior arts cannot accommodate such an increased speed.